1. Field of the Invention
The present invention relates to an image forming apparatus such as a laser beam printer.
2. Description of the Background Art
In an image forming apparatus (a laser beam printer and the like), a large amount of heat is given by a fixing unit or the like, to thereby fix a toner image formed on paper to the paper. Then, the paper having passed through the fixing unit further passes through a conveyance roller pair for output, to be outputted on a paper output tray.
In continuous printing wherein a plurality of printed matters are continuously outputted, the plurality of printed matters (sheets of paper) are stacked on the paper output tray. At that time, the sheets of paper on the paper output tray have considerably high temperature. For this reason, there occur some phenomena where a toner image on a sheet of paper among the plurality of stacked sheets of paper is adhered to another sheet of paper, where adjacent sheets of paper are adhered to each other through the mediation of a toner image on one sheet of paper, and the like. These phenomena may cause degradation of the image and are considered to be disadvantageous.
In order to avoid these phenomena, there is a technique of blowing air to sheets of paper having been subjected to a fixing process in order to cool these sheets of paper. For example, an image forming apparatus disclosed in Japanese Patent Application Laid Open Gazette No. 2000-95414 (Patent Document 1) comprises a cooling fan for blowing air to sheets of paper to be outputted to a paper output tray. In Patent Document 1, the amount of air to be blown from the cooling fan is controlled to be relatively large when OHP sheets are outputted and the amount of air to be blown from the cooling fan is controlled to be relatively small when sheets of plain paper are outputted.
In an image forming apparatus such as a laser beam printer, sheets of paper having different thickness, such as plain paper and thick paper, are used.
In printing of thick paper, generally, by controlling the fixing speed to be relatively low, a sufficient amount of heat can be given to the sheet of paper (see e.g., Japanese Patent Application Laid Open Gazette No. 9-281833 (Patent Document 2)).
When the fixing speed is set to be low in printing of thick paper, however, the image forming speed also becomes low.
Therefore, in order to suppress a decrease in the image forming speed, it is preferable that the fixing process should be performed, accompanying high-speed conveyance.
Also in such a fixing process accompanying high-speed conveyance, in order to avoid degradation of an image due to adhesion of the image or the like, there is a possible case where air is blown to cool the sheet of paper.
The inventor of the present invention has learned, however, that when the same amount of air is blown to both the thick paper and the plain paper in the fixing process accompanying high-speed conveyance, either one type of paper cannot be appropriately cooled. The inventor also has learned that even if the amount of air to be blown from the cooling fan is controlled to be small when sheets of plain paper which are relatively thin are outputted, it is not possible to perform appropriate cooling.